1. Field of the Invention
The present invention relates to electronic units ensuring on the one hand the function of a telephone set and on the other the functions of a computer terminal.
2. Description of the Prior Art
Telephone sets generally comprise a bell circuit, a telephone interface, a dialling keyboard and a telephone handset. The dialling keyboard comprises on the one hand decimal keys and, on the other, function keys for example for automatic recall of the last number dialled. The telephone interface comprises a first input connected to the dialling or control keyboard, a second input with two conductors connected to the telephone line through switches controlled by the position of the handset, a third input connected to the telephone handset. The telephone interface ensures the functions of transmission over the telephone line of dialling pulses controlled by the dialling or control keyboard, the functions of modulation transmission between the line and the handset, the switching off of the bell modulation circuit during dialling. The telephone interface is supplied by the telephone line alone. When the handset is lifted up, the switches are closed and the power supply is provided by the second input of the telephone interface.
Computer terminals comprise a local power supply connected to the main power supply generally through an insulating transformer. They comprise a keyboard connected to man-machine interface equipment. A part of the keyboard is formed of decimal keys, like the keyboard of the telephone set, another part of the keyboard comprising alphabetic keys and function keys.
Attempts have already been made to couple a telephone set with a computer terminal for providing more especially a single work station offering the possibility, from a keyboard comprising a single decimal part, of dialling telephone numbers or for dialoguing with the computer. Such work stations further provide direct dialogue between the telephone line and the computer. However, the design of such work stations raises a problem of galvanic insulation between the telephone line and main power supply. In fact, the decimal keyboard of the telephone set must be necessarily controlled by the telephone interface, itself supplied with power by the line when the handset is lifted. But the use of this decimal keyboard when the handset is lifted, for controlling the man-machine interface equipment, requires the telephone interface be supplied by a main power supply. When the telephone handset is lifted, the telephone interface then provides an electrical connection between the main power supply and the telephone line, which is incompatible with the insulation requirements of several thousands of volts between the telephone line and the main power supply. This insulation, made compulsory by all the standards relating to telephone apparatus, is intended to withstand the potential differences between the ground of the electrical main power supply and the ground of the telephone line network.
Attempts have been made to solve this problem by providing two separate keyboards, one for the telephone interface and the other for the man-machine interface equipment. That leads to increasing the number of keys and complicating the use of the work station. A composite keyboard has also been proposed comprising a single series of digital keys but two insulated series of switches, one series of which controls the telephone interface and the other the man-machine interface equipment. These constructions lead to complex and expensive devices.
The present invention aims more especially at avoiding the disadvantages of the known devices by proposing a telephone set--computer terminal assembly with a single digital keyboard and ensuring sufficient insulation between the telephone line and the main power supply. A single keyboard, comprising a number of common digital keys managed by the telephone interface, allows equally well dialoguing with the computer or generation of telephone commands.